Organic materials with controlled structures may have electronic and opto-electronic properties similar to inorganic semiconductor materials and are suitable for the fabrication of electronic and opto-electronic devices [J. H. Burroughes, C. A. Jones, and R. H. Friend, “New Semiconductor Device Physics in Polymer Diodes and Transistors,” Nature, 335, 137(1988)]. The organic semiconductor devices which have been investigated recently include but not limited to organic thin film transistor, organic photo-voltaic for solar electricity or photo-detector, organic solid state laser or organic solid state lighting, organic thin film memory for data storage, organic sensor for bio-application and chemical detection, and organic light emitting diode for flat panel applications.
Due to chemical nature, these electro-opto active organic semiconductor compounds are classified into two general categories: small molecules or macromolecules. Examples of small molecules include Alq3 [C. W. Tang and S. A. Van Slyke, “Organic Electroluminescent Diodes,” Applied Physics Letter, 51, 913(1988)], Irppy [M. Pfeiffer, S. R. Forrest, K. Leo and M. E. Thompson, “Electrophosphorescent p-i-n Organic Light-Emitting Devices for Very High Efficiency Flat Panel Displays,” Advanced Materials, 14(2), 1633(2002)], etc. which have been employed as light emitting materials or charge transporting materials in the fabrication of organic light emitting device (OLED), and the devices based on these small molecules are specifically referred to as SMOLEDs. Similarly, macromolecules such as poly(p-phenylene vinylene (PPV) [J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, “Light-Emitting Diodes Based on Conjugated Polymers,” Nature 347, 539 (1990)], polyfluorenes (PF) [A. Yasude, W. Knoll, A. Meisel, T. Miteva, D. Neher, H. G. Nothofer, and U. Scherf, “End-capped polyfluorenes, films and devices based thereon,” EPI 149 827, 2001], polyvinylcarbazole (PVK), polythiophenes (PT), etc. have been also employed as light emitting materials or charge transporting materials in the fabrication of OLEDs. The devices based on these polymers are specifically referred as PLEDs. For SMOLEDs, the organic semiconductor materials are normally deposited by a vacuum deposition method, also called a dry-vacuum processing in general. For PLEDs, the organic semiconductor polymers are deposited by spin coating, ink jet printing or screen printing, also called a wet-solution processing in general. Of the two OLED technologies, the industry considers polymer-based one to be a more viable solution because of the potentially lower manufacturing costs and greater processing flexibility. The main advantage of polymeric materials is that they can be deposited by simple solution processing techniques such as spin coating, inkjet printing and other conventional printing methods, which require relatively inexpensive equipment than the dry-vacuum processing. If organic semiconductor devices and circuits can be fabricated using the simple solution processing techniques, then low cost electronic and opto-electronic units and systems will be available in the future.
However, such an advantage in the fabrication of polymer-based devices (PLED for example) comparing to that of small molecular-based devices (SMOLED for example) has not been realized yet in industry. This is mainly due to the difficulty in fabricating good quality electrodes. In order to make an organic semiconductor device, electrodes must be deposited onto the organic semiconductor materials. Currently, no matter if the organic semiconductor devices are prepared from small molecules or polymers, their contact electrodes are still practically deposited by vacuum methods such as vacuum thermal evaporation, vacuum sputtering, chemical vapour deposition and others.
The requirements for electrode fabrication using the vacuum methods make the fabrication of polymer-based organic semiconductor devices to be even more complicated. For instance in making PLEDs, after the organic semiconductor materials have been deposited by a wet-solution method, the sample has to be transferred to a dry-vacuum environment for the deposition of contact electrodes. An integration of a wet environment and a dry-vacuum environment required in the current fabrication method is expensive and complicated.
Furthermore, in order to facilitate the charge injection in an organic semiconductor device, a low work function metallic layer is often used as cathode. One example is in an organic light-emitting device (OLED), where a low work function metal electrode such as Ca, Mg, Li and alloys must be deposited in order to achieve efficient charge injection. During operation, the low work function materials will allow a large injection rate of electrons into the organic semiconductor to increase the charge injection efficiency. Due to the chemical reactivity, most of the low work function materials are very active when exposed to an environment containing oxygen and water. These materials can form oxides or hydrides rapidly even at room temperature. Therefore, the fabrication of organic devices involving the low work function materials is even more difficult to achieve without a high vacuum. As mentioned before, it is rather expensive to set-up and to maintain a high vacuum system for the deposition of the low work function electrodes. This is especially true for large-scale production of these organic semiconductor devices for circuit applications. From the above comments, it is clear that it is highly desirable to have an organic device where the electrodes, specifically the ones with low work functions, are fabricated using a method other than vacuum deposition.